Apparently, according to the Princeton Dictionary of Buddhism, Nanda was the son of ŚUDDHODANA and MAHĀPRĀJĀPATĪ and half brother of the Buddha. He was a few days younger than the Buddha, and Mahāprajāpatī handed him over to a wet nurse so that she could raise the bodhisattva as her own son when the latter’s mother,MAHĀMĀYĀ, died. - Tsondru
I think I'll leave things as they are for the moment as I see that the English translation of The Words of My Perfect Teacher pp.95-96 mentions Nanda as the Buddha's cousin. I'm not sure that's correct but what to do. - Ts.